1. Field of the Invention
The present invention relates to a diaphragm control apparatus of a camera, and, more particularly, to a diaphragm control apparatus driving a diaphragm mechanism using a stepping motor.
2. Description of the Related Art
Since a stepping motor rotates by an angle proportional to the number of input pulses, the position control for the rotor can be done by an open-loop control and without using feedback control. This can simplify the required control circuit and results in a low-cost diaphragm control apparatus. Prior art which employs such a stepping motor for controlling the diaphragm of a camera is disclosed in, for example, Japanese Patent Disclosures No. 52-51937 and No. 58-82228 and U.S. Pat. Nos. 4,299,459 and 4,344,678.
Although not disclosed in these documents, there is a big problem in using a stepping motor which is originated from a step-out or out-of-step phenomenon. The step-out phenomenon is such that a load torque becomes greater than a driving torque of a stepping motor so that the rotor cannot follow a change in excitation of the stator, thus stopping the motor.
In a diaphragm control apparatus using a stepping motor, the step-out phenomenon is likely to occur when the load torque of a diaphragm mechanism locally or entirely increases due to a change in operational atmospheric conditions or when a battery voltage rapidly drops at the time of continuous photographing or under a low temperature, sot hat a sufficient exciting current for driving the motor cannot be supplied. Upon occurrence of the step-out phenomenon, the diaphragm stays stopped down or its movement stops in the middle of action, thus disabling a TTL type auto focus or deviating the brightness at the time of full-open photometry from the proper value as well as inhibiting the subsequent photographic operation.